JP5228627B2 - Switching power supply - Google Patents

Description

  The present invention relates to an insulation-type switching power supply device including an active clamp circuit.

The DC-DC converter disclosed in Patent Document 1 is an active clamp type DC-DC converter, and aims to reduce noise and increase efficiency. In this DC-DC converter, a series circuit including a primary side winding of a transformer and a first switch element is connected to a DC power source, and a capacitor and a second switch element are connected to both ends of the primary side winding of the transformer. An active clamp circuit consisting of is connected.
In the above active clamp type DC-DC converter, the first switch element that supplies power from the DC power supply to the transformer is turned off in the on state, and then the capacitor is connected via the parasitic diode of the second switch element. A switching operation is performed in which the second switch element is turned on in synchronization with the start of charging, and the second switch element is turned off after a predetermined time has elapsed.

JP 2007-97379 A

In an isolated switching power supply device including an active clamp circuit, in an equilibrium state (steady state), a first switch element that causes a current to flow through the primary winding of the transformer to supply power to the secondary side of the transformer, The second switch elements constituting the active clamp circuit that regenerates the exciting current flowing in the secondary winding to reset the primary winding are alternately turned on.
When the first switch element is turned off and the first switch element is turned off in a state where the excitation current supplied from the power source flows to the primary winding of the transformer, the excitation current flowing in the primary winding. Flows into the capacitor constituting the active clamp circuit via the parasitic diode of the second switch element. As a result, the capacitor is charged to increase its terminal voltage, and the excitation current of the primary winding decreases. Thereafter, when the second switch element is turned on, a series circuit of the primary side winding and the capacitor is formed, and the energy charged in the capacitor is transferred to the primary side winding. That is, the exciting current of the primary winding of the transformer further decreases, and the primary winding is reset. When the second switch element is turned off after the predetermined time has elapsed and the first switch element is turned on, the excitation current supplied from the power source flows again in the primary winding.

  On the other hand, in a non-equilibrium state such as when the load current becomes small, the input power supply becomes large, or when overcurrent protection functions, the first switch element has an on-time compared to an on-time in the balanced state. It is controlled to become smaller. That is, the on-time of the first switch element in one cycle is shortened, and the on-time of the second switch element is lengthened.

  In the non-equilibrium state, when the on-time of the second switch element becomes long, the energy charged in the capacitor is greatly moved to the primary winding of the transformer as compared with the case of the equilibrium state. . That is, as the second switch element is turned on, the exciting current of the primary winding of the transformer decreases, and then the current direction is reversed and current starts to flow from the capacitor toward the primary winding. That is, when the capacitor is discharged, a discharge current in the direction opposite to the excitation current fed from the power source flows through the primary winding of the transformer. If the second switch element is turned off in this state, the discharge current continues to flow through the primary winding via the parasitic diode of the first switch element.

  Depending on the operating conditions such as the switching duty and switching period of each switch element in the unbalanced state and the duration of the unbalanced state, the discharge current from the capacitor may increase. For example, if the non-equilibrium state in which the on-time of the first switch element decreases transiently and the on-time of the second switch element increases accordingly is continued for a plurality of cycles, the discharge current increases every cycle. In this case, there is a possibility that the magnetic saturation of the transformer may occur due to the increased discharge current, and problems such as an increase in power loss and an increase in heat generation due to the magnetic saturation occur.

  The present invention has been proposed in view of the above problems, and in a non-equilibrium state where the switching duty of the switch element changes transiently with a state change, the discharge current from the clamp capacitor constituting the active clamp circuit is It is an object of the present invention to provide a switching power supply device that can detect an increase and control the on / off state of a switch element so that magnetic saturation of a transformer does not occur.

A switching power supply according to the present invention includes a transformer and a first switch element connected in series to a primary side winding of the transformer and forming a current path for supplying power from an input power source to the primary side winding of the transformer. An active clamp circuit having at least a clamp capacitor and a second switch element for resetting the primary side winding of the transformer, and a secondary side output connected to the secondary side winding of the transformer A rectifying / smoothing circuit; and a control unit that performs on / off control of the first switch element and the second switch element. In a steady state, the control unit is configured such that the secondary output of the transformer has a target value. The target on-time of the first switch element in one cycle is set so as to be equal to and other than the target on-time of the first switch element in the one cycle The first switch element and the second switch are set according to the target on-time of the second switch element, and the target on-time of the first switch element and the target on-time of the second switch element are set. A switching power supply device that alternately turns on an element with a dead time interposed between the primary winding of the transformer and the first switch element in the current path , and the primary of the transformer A current detection unit that detects a reverse current flowing from the input power source to the first switch element via the input power source and flowing in the direction opposite to the direction of power feeding from the input power source to the primary winding of the transformer The control unit is configured to set an ON time of the second switch element in the next cycle when the reverse current detected by the current detection unit exceeds a threshold value. It is to be shorter than the target ON time of the second switching element.

  Here, “the reverse current flowing in the direction opposite to the feeding direction from the input power supply to the primary winding of the transformer” means that the on-time of the first switch element decreases transiently as the state changes. In the non-equilibrium state in which the ON time of the second switch element increases accordingly, the current flowing due to the discharge from the clamp capacitor provided in the active clamp circuit is shown. The second switch element of the active clamp circuit and the first switch element as the main switch are controlled so as to be alternately turned on within one switching cycle. In the equilibrium state (steady state), the ratio of the ON times of the first switch element and the second switch element is balanced, and excitation and reset of the primary winding are efficiently performed. On the other hand, in the non-equilibrium state in which the on-time of the first switch element decreases, the on-time of the second switch element in the switching cycle becomes long. Therefore, when the non-equilibrium state continues over a plurality of switching cycles, the discharge current flowing from the clamp capacitor provided in the active clamp circuit via the second switch element increases every switching cycle, and the primary of the transformer The reverse current flowing in the side winding increases with each switching period.

  Therefore, according to the present invention, in the non-equilibrium state in which the on-time of the first switch element decreases transiently with a state change such as a decrease in load current, an increase in input power supply, or an overcurrent limit, The excessive reverse current flowing to the primary side according to the discharge current is detected, and the on-time of the second switch element in the next cycle is appropriately controlled according to the detection result. It does not occur. That is, the switching power supply device can be protected.

  In the switching power supply according to the present invention, the control unit further maintains the first switch element in the off state in the next cycle when the reverse current detected by the current detection unit exceeds the threshold value. The second switch element is maintained in the off state. Thereby, it is possible to more effectively prevent an excessive reverse current from flowing to the primary side of the transformer, that is, it is possible to prevent magnetic saturation of the transformer.

  Moreover, in the switching power supply according to the present invention, the current detection unit is connected between a current transformer in which the primary winding is inserted on the current path and a terminal of the secondary winding of the current transformer. And a first resistance element for resetting the secondary winding, and a voltage comparator for detecting a voltage across the terminals of the first resistance element and comparing it with a reference value.

  As a result, the reverse current flowing on the current path on the primary side of the transformer also flows in the primary side winding of the current transformer. Accordingly, a current is also induced in the secondary winding of the current transformer by the reverse current. This excessive reverse current can be detected as a voltage value between the terminals of the first resistance element, which is a resistance for resetting the secondary winding of the current transformer.

  Furthermore, in the switching power supply according to the present invention, the rectifying element having one end connected to one terminal of the secondary winding of the current transformer, the other end of the rectifying element and the other side of the secondary winding of the current transformer. The second resistance element is connected to the terminal, and the second resistance element detects a current flowing in the feeding direction from the input power source to the primary side winding of the transformer.

  As a result, the current transformer includes the first resistance element, and can detect the reverse current flowing through the primary side winding of the transformer, and the second resistance element supplies the current to the primary side winding of the transformer. It is possible to detect a current flowing in the power feeding direction from the input power supply. Here, the current flowing in the feeding direction from the input power source to the primary winding of the transformer is a current in an equilibrium state. Therefore, it is possible to detect the current flowing through the primary winding of the transformer in both the unbalanced state and the balanced state.

  According to the switching power supply device of the present invention, in the non-equilibrium state in which the on-time of the first switch element decreases transiently with the state change and the on-time of the second switch element increases accordingly, the active clamp circuit It is possible to detect an increase in the discharge current from the clamp capacitor that constitutes and to control the on / off state of the switch element so that magnetic saturation of the transformer does not occur.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit block diagram of a switching power supply 1 constituting a forward converter according to an embodiment of the present invention.

The switching power supply 1 includes a transformer T having a primary side winding L1 and a secondary side winding L2.
First, the primary side circuit of the transformer T will be described. The positive side terminal of the input power source E is connected to the primary side winding L1 of the transformer T and one end of the clamp capacitor C2. The other end of the primary side winding L1 of the transformer T is connected to the source terminal of the MOS transistor Q2 as the second switch element and one end of the primary side winding LC1 of the current transformer CT. The other end of the primary side winding L1 of the transformer T is connected to the drain terminal of the MOS transistor Q1, which is the first switch element, via the primary side winding LC1 of the current transformer CT. The source terminal of the MOS transistor Q1 is connected to the negative terminal of the input power supply E. The other end of the clamp capacitor C2 and the drain terminal of the MOS transistor Q2 are connected.

  A control signal from the control unit CNT is input to the gate terminals of the MOS transistors Q1 and Q2, thereby controlling on / off driving of the MOS transistors Q1 and Q2. This control signal is a signal repeated at a predetermined cycle, and alternately turns on and off the MOS transistors Q1 and Q2. That is, the MOS transistor Q1 is maintained in the on state at a predetermined time within one cycle, and the MOS transistor Q2 is maintained in the off state during that time. The MOS transistor Q1 is maintained in the off state for the remaining time within one cycle after the lapse of the predetermined time. In the meantime, the MOS transistor Q2 is kept on. The control unit CNT performs control by providing a dead time so that the MOS transistor Q1 and the MOS transistor Q2 do not turn on at the same time.

  When the MOS transistor Q1 is turned on, the input power source E is connected to the primary side winding L1, and the primary side winding L1 is excited by the potential of the input power source E. The current flowing through the primary side winding L1 increases with the time that the MOS transistor Q1 is on. In this state, power from the input power source E is supplied to the primary winding L1.

  The current that flows to the primary side when the MOS transistor Q1 is turned on is constituted by the sum of the exciting current that excites the primary winding L1 and the load current that is propagated to the load side. Even when the MOS transistor Q1 is turned off, the exciting current remains in the primary winding L1 and continues to flow. Accordingly, it is necessary to reset the excitation of the primary winding L1 before the MOS transistor Q1 is turned on in the next switching cycle and new power is supplied to the primary winding L1. In this embodiment, an active clamp method is employed for resetting the primary winding L1. Specifically, an active clamp circuit is configured by connecting a series circuit of a MOS transistor Q2 and a clamp capacitor C2 to both ends of the primary winding L1. The primary winding L1 is reset by turning on the MOS transistor Q2.

  When the MOS transistor Q1 in the on state is turned off, the exciting current flowing in the primary side winding L1 flows into the clamp capacitor C2 via the parasitic diode of the MOS transistor Q2, and charging of the clamp capacitor C2 is started. . As a result, the clamp capacitor C2 is charged to increase its terminal voltage, and the exciting current of the primary winding L1 decreases. Thereafter, when the MOS transistor Q2 is turned on, a series circuit of the primary side winding L1 and the clamp capacitor C2 is formed, and the energy charged in the clamp capacitor C2 is moved to the primary side winding L1. That is, the exciting current of the primary side winding L1 of the transformer T further decreases, and the primary side winding L1 is reset. Then, after a predetermined time has elapsed, the MOS transistor Q2 is turned off. After that, when the MOS transistor Q1 is turned on, the exciting current supplied from the input power source E flows again in the primary winding L1.

  Next, the secondary side circuit of the transformer T will be described. A rectifying / smoothing circuit is connected to the secondary winding L2 of the transformer T, and a current (secondary output) is supplied from the rectifying / smoothing circuit to a load (not shown). Hereinafter, the rectifying / smoothing circuit will be described in detail.

  One end of the secondary winding L2 of the transformer T is connected to the anode terminal of the rectifier diode element D1. The cathode terminal of the rectifier diode element D1 is connected to one end of the choke coil L and the cathode terminal of the rectifier diode element D2. The other end of the choke coil L is connected to the output terminal on the high level side and one end of the smoothing capacitor C1. The smoothing capacitor C1 is connected between the two output terminals. On the other hand, the other end of the secondary winding L2 is connected to the anode terminal of the rectifier diode element D2, the other end of the smoothing capacitor C1, and the output terminal on the low level side.

  This embodiment illustrates the configuration of a forward converter. Therefore, during the period when the MOS transistor Q1 is turned on and power is supplied from the input power source E to the primary side winding L1, one end of the secondary side winding L2 is at a higher voltage level than the other end. Become. That is, a current flows through the rectifier diode element D1. This current is supplied via the choke coil L to the smoothing capacitor C1 and the output terminal on the high level side.

  When the MOS transistor Q1 is turned off and the power supply from the input power source E to the primary winding L1 is cut off, the electromotive force induced in the secondary winding L2 is cut off by the diode element D1. As a result, the current flowing through the choke coil L continues to flow through the diode element D2, and releases the electromagnetic energy accumulated so far to the output side.

  The MOS transistors Q1 and Q2 are on / off controlled by a control signal output from the control unit CNT so that the output voltage VO becomes a desired set value. Specifically, the switching duty, that is, the so-called on-time of turning on the MOS transistor Q1 is adjusted within a certain switching period.

Next, a detection unit that detects a current flowing through the primary circuit will be described. In the embodiment, two types of current detection units are provided.
The first detection unit outputs a detection signal VD1. The first detection unit detects a reverse current flowing in the primary circuit in the direction opposite to the feeding direction from the input power source E. The detection signal VD1 is input to the control unit CNT. When the detection signal VD1 exceeds the threshold value, the ON time of the MOS transistor Q2 in the next cycle is shorter than the set target ON time of the MOS transistor Q2 by the control unit CNT. Thus, the off timing of the MOS transistor Q2 is controlled. When the detection signal VD1 greatly exceeds the threshold value, the MOS transistor Q1 is maintained in the off state and the MOS transistor Q2 is maintained in the off state in the next cycle.
The second detection unit outputs a detection signal VD2. The second detection unit detects a current flowing in the primary side circuit in the power feeding direction from the input power source E, that is, a sum of an excitation current and a load current.

The detection unit is configured by a current transformer CT.
The first detection unit includes a current transformer CT, a reset resistance element R1, and a detector VD. The second detection unit includes a current transformer CT, a reset resistance element R1, diode elements D3 and D4, a sense resistance element R2, a resistance element R3, and a capacitance element C3. Here, the current transformer CT and the reset resistance element R1 are common to both detection units.

  One end of the secondary winding LC2 of the current transformer CT is connected to one end of the reset resistance element R1 and the anode terminal of the diode element D3, and is input to the detector VD. The cathode terminal of the diode element D3 is connected to one end of the sense resistor element R2 and the anode terminal of the diode element D4. The cathode terminal of the diode element D4 is connected to one end of the resistance element R3. The other end of the resistive element R3 is connected to the capacitive element C3. The other end of the secondary winding LC2 of the current transformer CT is connected to the other end of the reset resistance element R1, the other end of the sense resistance element R2, and the other end of the capacitive element C3, and is input to the detector VD. ing. A second detection signal VD2 is output from the connection point between the resistor element R3 and the capacitor element C3.

  A current corresponding to the current supplied to the primary winding L1 of the transformer T flows through the sense resistor R2. Since the current supplied to the primary side winding L1 of the transformer T flows to the primary side winding LC1 of the current transformer CT at the same time, the sense resistance element R2 is connected from the secondary side winding LC2 via the diode element D3. Because it is biased. The value converted into a voltage and detected by the sense resistor element R2 is integrated by the resistor element R3 and the capacitor element C3 via the diode element D4. The second detection signal VD2 is obtained by integrating or peak-holding the detection value of the sense resistor element R2 that varies with time.

  The reset resistor element R1 is a resistor that resets the exciting current of the secondary winding LC2 of the current transformer CT. After the MOS transistor Q1 is turned off and the current supplied to the primary side winding L1 of the transformer T is cut off, the secondary side winding LC2 of the current transformer CT induces an electromotive force in the direction cut off by the diode element D3. Is done. A current based on this electromotive force flows through the reset resistance element R1, and the current transformer CT is reset.

  The first detection unit actively uses the reset resistance element R1 for resetting the current transformer CT. In a non-equilibrium state such as when the load current becomes small, the input power source becomes large, or the overcurrent protection functions, the MOS transistor Q1 has an on-time that is smaller than the on-time in the balanced state. Controlled. That is, the ON time of the MOS transistor Q1 in one cycle is shortened, whereas the ON time of the MOS transistor Q2 is increased. Thereby, the discharge from the clamp capacitor C2 of the active clamp circuit becomes larger than that in the equilibrium state, and the energy of the clamp capacitor C2 is greatly moved to the primary winding L1 of the transformer T. The exciting current of the primary side winding L1 of the transformer T decreases, and then the current direction is reversed, and a reverse current starts to flow from the clamp capacitor C2 toward the primary side winding L1. If the switching operation shifts to the next cycle and the MOS transistor Q2 is turned off while the reverse current is flowing, the MOS transistor Q1 is turned on thereafter through the parasitic diode of the MOS transistor Q1. The reverse current corresponding to the discharge current from the clamp capacitor C2 continues to flow through the primary winding L1. As a result, a reverse current flows through the primary winding LC1 of the current transformer CT.

  That is, the reverse current flows from one end of the primary winding L1 through the input power source E from the positive terminal to the negative terminal, and flows through the antiparallel diode of the MOS transistor Q1. The reverse current flows through the primary winding LC1 of the current transformer CT. The current direction is the current direction that flows when power is supplied from the input power source E to the primary winding L1 in a balanced state. The opposite direction.

  Due to this reverse current, an electromotive force is induced in the secondary winding LC2 of the current transformer CT in a direction in which the current is cut off by the diode element D3. Since this electromotive force is the same as the direction by the exciting current of the current transformer CT, a current flows through the reset resistance element R1. The detector VD detects the voltage across the reset resistor element R1 and outputs a first detection signal VD1.

  Here, since the reset resistance element R1 is provided for resetting the current transformer CT, it is preferable that a large voltage is generated between the terminals of the reset resistance element R1 during the reset operation. This is because a reverse bias is applied to the secondary winding LC2 to facilitate resetting. Therefore, the reset resistance element R1 is generally configured to have a high resistance value. For example, the sense resistance element R2 is several ohms to several tens of ohms, whereas the reset resistance element R1 is composed of several kiloohms.

  Next, with reference to FIGS. 2-6, operation | movement of the switching power supply device 1 (FIG. 1) of embodiment is demonstrated.

2 to 6 show operation waveforms of respective signals for the same operation, with time on the horizontal axis. FIG. 2 shows the exciting current of the transformer T. The feeding direction from the input power source E to the primary winding L1 is shown as positive. FIG. 3 shows the charging current to the clamp capacitor C2 of the active clamp circuit. FIG. 4 shows the voltage across the clamp capacitor C2 of the active clamp circuit. FIG. 5 shows the current flowing through the primary winding LC1 of the current transformer CT. The feeding direction from the input power source E to the primary winding L1 is shown as positive. FIG. 6 shows the voltage across the terminals of the reset resistance element R1. It is shown as the negative direction of the current that flows when resetting.
Although a dead time is provided so that the MOS transistor Q1 and the MOS transistor Q2 do not turn on at the same time, the time is very short and is not shown here.

  2 to 6, it is assumed that the state changes at time t0. That is, control is performed in an equilibrium state before time t0. A non-equilibrium state in which the switching duty decreases transiently after time t0 in response to the start of at least one of a decrease in load current, a rise in input power, and start of overcurrent limiting control at time t0. Shall be transferred to.

  First, the switching operation in an equilibrium state, that is, a steady state will be described.

  In the balanced state, as shown in FIG. 2, the exciting current flowing through the primary winding L1 of the transformer T is balanced within the switching period. That is, during the ON time of the MOS transistor Q1, the exciting current flows through the current path I1 (FIG. 1) together with the load current, and this exciting current increases with time. When the MOS transistor Q1 is turned off, the exciting current flowing in the primary winding L1 charges the clamp capacitor C2 of the active clamp circuit via the current path I2 (FIG. 1) via the parasitic diode of the MOS transistor Q2. This will decrease. Thereafter, when the MOS transistor Q2 is turned on, a series circuit of the primary side winding L1 and the clamp capacitor C2 is formed, and the energy charged in the clamp capacitor C2 is moved to the primary side winding L1 to be excited current. Will continue to decrease. Thereby, the primary winding L1 is reset. At the end of one cycle, the MOS transistor Q2 is turned off and the next switching cycle is started.

  In this case, as shown in FIG. 3, in the second half of the on-time of the MOS transistor Q2, there is a discharge from the clamp capacitor C2, and a reverse current flows toward the input power source E through the primary winding L1. However, this current is limited and is eliminated when the MOS transistor Q1 is turned on in the next switching cycle, and an exciting current flows in the current path I1 together with the load current (see FIG. 5).

  When a state change such as a decrease in load current, an increase in input power supply, or an overcurrent limit occurs at time t0, a switching power supply device 1 passes through a dead time period in which both MOS transistors Q1 and Q2 are turned off. The MOS transistor Q1 shifts to a non-equilibrium state where the switching duty of the transistor Q1 decreases transiently.

  In the non-equilibrium state, the on-time of the MOS transistor Q2 becomes longer than the on-time of the MOS transistor Q1. When the ON time of the MOS transistor Q2 becomes longer, the energy of the clamp capacitor C2 is greatly moved to the primary winding L1 of the transformer T. After the exciting current of the primary winding L1 decreases and the exciting current becomes zero, the direction of the current is reversed and discharge is started from the clamp capacitor C2. As a result, a reverse current flows from the clamp capacitor C2 toward the primary winding L1 via the current path I3 (FIG. 1). This current value increases with time (period Q1 (off) and Q2 (on) in FIG. 2).

When the next switching cycle is started and the MOS transistor Q2 is turned off, the reverse current flowing in the primary winding L1 continues to flow as an exciting current through the current path I4.
After that, when the MOS transistor Q1 is turned on, the input power source E is applied to the primary winding L1, and the reverse current decreases with time (period of Q1 (on) and Q2 (off) in FIG. 2). . However, in the non-equilibrium state, the switching duty (on time) of the MOS transistor Q1 is reduced compared to that in the equilibrium state, so the on-time of the MOS transistor Q1 is short. Before the reverse current due to the current path I4 is eliminated, the MOS transistor Q1 is turned off and the MOS transistor Q2 is turned on again. Then, the reverse current increases again via the current path I3 (period Q1 (off) and Q2 (on) in FIG. 2).

  Due to the discharge current from the clamp capacitor C2 of the active clamp circuit, the current supplied to the primary winding L1 increases during the ON time of the MOS transistor Q2 for each switching period (see FIG. 3). In response to this, the charging voltage charged in the clamp capacitor C2 decreases (see FIG. 4).

  Then, when the MOS transistor Q2 is turned off, a reverse current flows through the primary circuit, so that a reverse current flows through the primary winding LC1 of the current transformer CT via the current path I4. This current increases with the switching period while the non-equilibrium state continues (see FIG. 5). As a reverse current flows through the primary winding LC1 of the current transformer CT, a current flows through the secondary winding LC2 of the current transformer CT via the current path I5. Since the reverse current increases with the switching period, the current flowing through the secondary winding LC2 also increases with the switching period. This current flows through the reset resistance element R1, and the detection voltage increases with the switching period (see FIG. 6). If the threshold voltage (VT) detected by the detector VD is set, it can be detected by the first detection signal VD1 that the current value of the reverse current has reached a predetermined value.

In response to the first detection signal VD1, the control unit CNT limits the ON time of the MOS transistor Q2 in the next period. That is, the MOS transistor Q2 is controlled so that the actual on-time is shorter than the target on-time. The detection voltage by the detector VD at this time is as shown in FIG. The detection voltage is limited to a threshold value (VT). The exciting current flowing in the primary winding L1 of the transformer T at this time is as shown by the dotted line in FIG. By reducing the ON time of the MOS transistor Q2, the reverse current is limited.
In addition, when the voltage detected by the detector greatly exceeds the threshold value, the control unit CNT performs control so that the switching operation itself of both the switches of the MOS transistors Q1 and Q2 is stopped. From this, in a non-equilibrium state accompanying a transient decrease in the switching duty, the reverse current flowing in the direction opposite to the direction of feeding from the input power source E to the primary winding L1 of the transformer T increases. Can be prevented.

  Here, the current path I1 is an example of a current path that feeds power from the input power source E to the primary winding L1 of the transformer T. The MOS transistor Q1 is an example of a first switch element, and the MOS transistor Q2 is an example of a second switch element. The current transformer CT, the reset resistance element R1, and the detector VD are examples of a current detection unit. The reset resistance element R1 is an example of a first resistance element, and the sense resistance element R2 is an example of a second resistance element. The detector VD is an example of a voltage comparator.

  As described above, according to the embodiment of the present invention, the switching power supply device 1 is connected in series to the transformer T and the primary side winding L1 of the transformer T, and the primary side winding L1 of the transformer T. Are provided with a MOS transistor Q1 that forms a current path I1 that feeds power from the input power source E, and an active clamp circuit that resets the primary winding L1 of the transformer T. Here, the MOS transistor Q1 and the MOS transistor Q2 included in the active clamp circuit are alternately turned on / off using a switching cycle as an operation unit. A current transformer CT for detecting a reverse current (current flowing through the current path I4) in a direction opposite to the direction of power feeding from the input power source E to the primary winding L1 of the transformer T, and a reset resistance element R1; A detector VD is provided. In addition, a control unit CNT that performs on / off control of the MOS transistor Q1 and the MOS transistor Q2 is provided. In a steady state, the control unit CNT sets the target on time of the MOS transistor Q1 and the target on time of the MOS transistor Q2 in one cycle so that the secondary output of the transformer T becomes the target value, and the set target on time is set. Control is performed so that the MOS transistor Q1 and the MOS transistor Q2 are alternately turned on according to time. Further, when the reverse current exceeds the threshold value, control is performed so as to shorten the ON time of the MOS transistor Q2 in the next period.

  As a result, it is possible to detect a reverse current that flows in a direction opposite to the feeding direction from the input power source E to the primary winding L1 of the transformer T.

  Due to a discharge from the clamp capacitor C2 provided in the active clamp circuit in a non-equilibrium state in which the switching duty decreases transiently with a change in state such as a decrease in load current, an increase in input power supply, or an overcurrent limit. Thus, the reverse current flowing through the primary winding L1 in the direction of the current path I4 can be detected.

In the non-equilibrium state, it is possible to detect an increase in the reverse current of the primary winding L1 due to a long on-time of the second switch element in the switching period. When the reverse current exceeds the threshold, the on-time of the MOS transistor Q2 in the next cycle is forcibly limited to be shorter than the set target on-time, so the reverse current flowing in the primary winding L1 Can be prevented from increasing. Since the control is performed so that the magnetic saturation of the transformer T does not occur, the switching power supply device can be effectively protected.
Further, since the MOS transistor Q1 is turned off together with the MOS transistor Q2 being turned off, it is possible to more effectively prevent the reverse current from flowing into the primary side winding L1 of the transformer T.

  A current transformer CT, a reset resistor element R1, and a detector VD are included to constitute a first detector, and a current transformer CT, a reset resistor element R1, diode elements D3 and D4, a sense resistor element R2, and a resistor element R3. , And the capacitive element C3, the second detection unit can be configured. In this case, the current transformer CT and the reset resistance element R1 can be shared by both detection units.

  Thereby, the reverse current flowing through the current path I4 also flows through the primary winding LC1 of the current transformer CT. Therefore, a current is also induced in the secondary winding LC2 of the current transformer CT by the reverse current, and an excessive reverse current can be detected as a voltage value between the terminals of the reset resistance element R1. In addition, the current transformer CT can detect a current flowing in the feeding direction from the input power source E to the primary winding L1 of the transformer T by the sense resistor element R2. Therefore, the current flowing through the primary side winding L1 of the transformer T can be detected in both the unbalanced state and the balanced state.

Needless to say, the present invention is not limited to the above-described embodiment, and various improvements and modifications can be made without departing from the spirit of the present invention.
For example, in the switching power supply device 1 of the embodiment, the configuration including the current transformer CT as the current detection unit has been described, but the present invention is not limited to this. Any configuration capable of detecting the reverse current flowing through the current path I4 may be used. For example, instead of the current transformer CT, a configuration including a shunt resistor element may be employed.
In the switching power supply device 1, the case where the MOS transistors Q 1 and Q 2 are turned on has been described for any current direction of the MOS transistors Q 1 and Q 2, but the present invention is not limited to this. In general, a MOS transistor has a structure in which an antiparallel diode is built in due to its device configuration. Therefore, for a current from a source terminal to a drain terminal, instead of the MOS transistors Q1 and Q2, or A configuration may be adopted in which current flows through the antiparallel diode together with the MOS transistors Q1 and Q2. Further, a diode element may be connected in parallel to the MOS transistors Q1 and Q2.

It is a circuit block diagram of an embodiment of the present invention. It is a wave form diagram which shows the exciting current of the primary side coil | winding of a transformer. It is a wave form diagram which shows the electric current which flows into the clamp capacitor | condenser of an active clamp circuit. It is a wave form diagram which shows the voltage between terminals of the clamp capacitor of an active clamp circuit. It is a wave form diagram which shows the electric current which flows through the primary side coil | winding of a current transformer. It is a wave form diagram which shows the voltage between terminals of a reset resistance element when not carrying out current limitation. It is a wave form diagram which shows the voltage between terminals of a reset resistance element in case current limitation is carried out.

DESCRIPTION OF SYMBOLS 1 Switching power supply device CNT Control part CT Current transformer T Transformer L1 Primary side winding L2 of transformer T Secondary side winding LC1 of current transformer T Primary side winding LC2 Secondary side winding of current transformer T Q1, Q2 MOS transistor R1, reset resistance element R2, sense resistance element VD detector

Claims (4)

With a transformer,
A first switch element connected in series to the primary winding of the transformer and forming a current path for supplying power from an input power source to the primary winding of the transformer;
An active clamp circuit having at least a clamp capacitor and a second switch element for resetting the primary winding of the transformer;
A rectifying / smoothing circuit connected to the secondary winding of the transformer and extracting the secondary output;
A controller that performs on / off control of the first switch element and the second switch element,
In a steady state, the control unit sets a target on-time of the first switch element in one cycle so that the secondary side output of the transformer becomes a target value, and the first switch element in the one cycle Other than the target ON time of the second switch element is set as the target ON time of the second switch element, and the first switch according to the set target ON time of the first switch element and the target ON time of the second switch element A switching power supply device that alternately turns on an element and the second switch element across a dead time ,
Provided between the primary winding of the transformer and the first switch element of the current path, from the primary side of the transformer to the first switch element via the input power supply , A current detection unit for detecting a reverse current flowing in a direction opposite to the feeding direction from the input power source to the primary winding of the transformer;
The control unit, when the reverse current detected by the current detection unit exceeds a threshold value, sets the on-time of the second switch element in the next cycle to the target on-state of the second switch element A switching power supply characterized by being shorter than time. When the reverse current detected by the current detection unit exceeds a threshold, the control unit maintains the first switch element in an off state and turns off the second switch element in a next cycle. The switching power supply device according to claim 1, wherein the switching power supply device is maintained. The current detector is
A current transformer in which a primary winding is inserted on the current path;
A first resistance element connected between terminals of the secondary winding of the current transformer for resetting the secondary winding;
3. The switching power supply device according to claim 1, further comprising: a voltage comparator that detects a voltage between terminals of the first resistance element and compares the voltage with a reference value. 4. A rectifying element having one end connected to one terminal of the secondary winding of the current transformer;
A second resistance element connected between the other end of the rectifying element and the other terminal of the secondary winding of the current transformer;
4. The switching power supply device according to claim 3, wherein the second resistance element detects a current flowing in a feeding direction from the input power supply to the primary side winding of the transformer. 5.

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